Servlet是如何实现MVC的

Servlet是一种服务器端的编程语言，是J2EE中比较关键的组成部分，Servlet技术的推出，扩展了Java语言在服务器端开发的功能，巩固了Java语言在服务器端开发中的地位，而且现在使用非常广泛的JSP技术也是基于Servlet的原理.

JSP+JavaBeans+Servlet成为实现MVC模式的一种有效的选择。



如果我们要实现一个对用户的增删改查，并且要求符合对扩展开发，对修改关闭的原则，该怎么做呢？

首先，这是我们的类图以及类与类之间调用的时序图。





根据UML图来实现代码：

TestServlet类

package com.bjpowernode.servlet;

import java.io.IOException;
import java.util.List;

import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

public class TestServlet extends HttpServlet {

protected void doGet(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
String requestURI = request.getRequestURI();
System.out.println("requestURI=" + requestURI);
String path = requestURI.substring(requestURI.indexOf("/",1),requestURI.indexOf("."));
System.out.println("path = " +path);

String username = request.getParameter("username");

Action action = null;
if("/servlet/delUser".equals(path)){
action = new DelUserAction();
}else if("/servlet/addUser".equals(path)){
action = new AddUserAction();
}else if("/servlet/modifyUser".equals(path)){
action = new ModifyAction();
}else if("/servlet/queryUser".equals(path)){
action = new QueryUserAction();
}else{
throw new RuntimeException("请求失败");
}
String forward ="";
try {
forward= action.execute(request, response);
} catch (Exception e) {
e.printStackTrace();
}
request.getRequestDispatcher(forward).forward(request, response);

}

protected void doPost(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
doGet(request,response);
}

}

抽象接口Action

package com.bjpowernode.servlet;

import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

public interface Action {

public String execute(HttpServletRequest request, HttpServletResponse response)
throws Exception;

}

添加类，这里就只写添加类了，其他类一样

package com.bjpowernode.servlet;

import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

public class AddUserAction implements Action {

public String execute(HttpServletRequest request,
HttpServletResponse response) throws Exception {
String username = request.getParameter("username");
//int age = Integer.parseInt(request.getParameter("username"));
//String sex = request.getParameter("sex");

//调用业务逻辑
UserManager userManager = new UserManager();
userManager.add(username);

return "/add_success.jsp";
}

}

Web.xml配置

<welcome-file-list>
<welcome-file>index.jsp</welcome-file>
</welcome-file-list>

<servlet>
<servlet-name>TestServlet</servlet-name>
<servlet-class>com.bjpowernode.servlet.TestServlet</servlet-class>
</servlet>
<servlet-mapping>
<servlet-name>TestServlet</servlet-name>
<!--
<url-pattern>/servlet/TestServlet</url-pattern>
-->
<url-pattern>*.do</url-pattern>
</servlet-mapping>

这里我们发现，在testAction类中，用到了大量的if和else，而当我们需要扩展一个上传类的时候，就需要修改if/else，所以，这就不符合对扩展开发，对修改关闭原则，所以，我们需要把这部分提取出来，配置到配置文件里，用反射进行配置。

我们需要先配置配置文件

<action-config>
<action path ="/servlet/delUser" type = "com.bjpowernode.servlet.DelUserAction">
<forward name = "success">del_success.jsp</forward>
<forward name = "error">del_error.jsp</forward>
</action>
<action path ="/servlet/addUser" type = "com.bjpowernode.servlet.AddUserAction">
<forward name = "success">add_success.jsp</forward>
<forward name = "error">add_error.jsp</forward>
</action>
<action path ="/servlet/modifyUser" type = "com.bjpowernode.servlet.ModifyAction">
<forward name = "success">modify_success.jsp</forward>
<forward name = "error">modify_error.jsp</forward>
</action>
<action path ="/servlet/queryUser" type = "com.bjpowernode.servlet.QueryUserAction">
<forward name = "success">query_success.jsp</forward>
<forward name = "error">query_error.jsp</forward>
</action>

</action-config>

需要获取actionMapping对象

ActionMapping{
private String path;
private String type;
Map forwardMap;

}
forwardMap{
key = "success";
value ="/del_success.jsp"
key ="error"
value ="del_error.jsp";

}

Map map = new HashMap();
map.put("/servlet/delUser",actionMapping);
map.put("/servlet/addUser",actionMapping);
map.put("/servlet/modifyUser",actionMapping);
map.put("/servlet/queryUser",actionMapping);

// 如果是删除ActionMapping存储如下：
actionMapping{
path ="/servlet/delUser";
type ="com.bjpowernode.servlet.DelUserAction"
forwardMap{
key ="success",value ="/del_success.jsp"
key ="error",value"/del_error.jsp"
}
}

最后根据获取的对象采用反射来动态实例化Action，达到我们的目的。

String path = "/servlet/delUser";

//  根据截取的URL请求，到Map中取得本次请求对应的Action
ActionMapping actionMapping =(ActionMapping)map.get(path);

//取得本次请求对应的Action类的完整路径
String type = actionMapping.getType();  //com.bjpowernode.servlet.DelUserAction

// 采用反射动态实例化Action
Action action  = (Action)class.forName(type).newInstance();

//动态待用Action中的execute方法
String forward = action.execute(request.response);
//根据路径完成转向
request.getRequestDispatcher(forward).forward(request, response);

这样，我们基本就是实现了对扩展开发，对修改封闭的原则，但是，这仅仅是一个功能我们就需要做这么多事情，写这么多代码，对程序员来说这是一项不小的工作量，那么我们有没有其他的办法让我们的工作量小一点呢？